The technical field of this invention is the control of understeer in motor vehicles having braking systems.
Many motor vehicles are provided with braking systems having yaw stability controls in which a yaw error signal, based on the difference between a calculated desired yaw rate and a sensed or estimated yaw rate, is used to determine and apply a correcting braking force to one or more of the vehicle wheels. Some of these systems specifically address correction of understeer conditions. When the motor vehicle brake controller has operator independent control of all four wheel brakes (a four channel braking system), the preferred corrective action in understeer is an increase in the brake force applied to an inside wheel, relative to the turn, a decrease in the brake force applied to the outside of the wheel, or a combination of the two, in order to produce an increase in yaw rate and thus counteract the understeer condition. In the absence of active anti-lock braking, the change in differential brake pressure is preferably applied to the rear axle. But hard turns can tend to produce a roll of the vehicle body that relatively decreases the normal force, and thus the lateral traction, of the tires on the inside of the turn. In addition, braking in such a hard turn can produce a forward pitch of the vehicle body that further decreases the normal force on the inside rear tire. If these motions produce sufficient roll and pitch, the ability of the inside rear wheel to provide understeer correction can decrease.
A vehicle brake control of the type providing understeer correction in response to the detection of yaw rate error is shown in the U.S. Pat. No. 5,746,486, entitled Brake Control System and issued to Ronald Paul et al on May 5, 1998. This system identifies and signals an understeer condition and provides corrective braking action with an increase in differential brake pressure favoring an inner wheel. The control shown is applicable to a four channel brake system and provides the understeer correction in such a system to the front wheels when ABS is active and to the rear wheels when ABS is not active. When an understeer condition exists with no ABS activity, the system may not produce the most efficient understeer correction, in extreme hard turns, when vehicle roll and/or forward pitch reduce the ability of the inner rear wheel to apply the correction.
The brake control apparatus of this invention is responsive to one or more sensors to detect in a vehicle turn a likely predetermined low traction on one of a pair of rear wheels on the inside of the turn to provide a low traction indicating signal and uses this signal to determine, when understeer correction is required, an increase in brake pressure on a wheel on the inside of the turn relative to a brake pressure on a wheel on the outside of the turn. In the absence of anti-lock braking activity, the increase is applied to the rear wheels in the absence of the low traction indicating signal but to the front wheels when the low traction indicating signal is present.
Preferred sensors include any vehicle related sensors that indicate reduced normal force on the inside rear wheel or a raised inside rear corner of the vehicle body, which will produce such a reduced normal force and thus a reduced traction compared with the front inside tire. Specifically, preferred sensors include a suspension position sensor for the inside rear wheel or other sensor derived information from a suspension control system that indicates large body roll in a turn together with forward body pitch. In the absence of a suspension control system, preferred sensors include vehicle lateral and longitudinal accelerometers indicating vehicle roll and pitch together with a steer angle sensor indicating a significant turn. An indication could also be derived from a normal force sensor on the wheel or normal force information derived from other sensors such as a tire pressure sensor.